Atrial fibrillation and ventricular tachyarrhythmias occurring in patients with structurally abnormal hearts are the most important arrhythmias in contemporary cardiology.1-3 they represent the most frequently encountered tachycardias, account for the most morbidity and mortality, and, despite much progress, remain therapeutic challenges. Invasive studies of the electrical activity of the heart (electrophysiologic studies) are often used in the diagnosis and therapy of arrhythmias and many arrhythmias can be cured by selective destruction of critical electrical pathways with radiofrequency (RF) catheter ablation. Attempts at applying ablation to atrial fibrillation and ventricular tachycardia have been made. Success has been limited, however, by the long time duration of procedures, resulting from the difficulty of creating continuous linear lesions in a setting where areas of ablated myocardium cannot be directly visualized. Continuous linear lesions, without gaps, can block critical arrhythmogenic circuits and reduce the amount of electrically contiguous arrhythmoginic substrate, thereby eliminating arrhythmias. We hypothesize that magnetic resonance imaging (MRI), with MRI-compatible diagnostic and therapeutic systems;can allow electrophysiology studies and catheter ablation to be performed without x-ray radiation. We also hypothesize that this technology will provide the ability to visualize ablation lesions, which should greatly simplify production of continuous linear lesions, and should improve the effectiveness of ablation procedures in general. In addition to electrophysiology, these methods may be applicable to guiding other diagnostic and therapeutic techniques. In Phase I, we completed a prototype steerable ablation catheter that has allowed us to target any area of the endocardial surface of the heart. We also developed integral filters for protecting the catheters from excessive heating during MR imaging. We tested the prototype catheters in animals to show that electrophysiology studies can be done under MR guidance alone, that lesions can be produced and imaged, that linear lesions can be produced, and that MRI has sufficient resolution to allow detection of significant gaps in the lesions. In Phase II, we will develop, test, and prepare for manufacturing and marketing an improved, clinical-grade version of the ablation system, and apply for FDA approval for testing the technology. The improved, clinical-grade catheter system will incorporate all capabilities previously tested individually. The improved catheter system will both ablate cardiac tissue and sense cardiac activity. The improved catheter will also have improved filtering and shielding to allow all current clinical MRI pulse sequences to be used for imaging. Further, the improved catheter system will have sensors incorporated into the tip of the catheters to allow accurate determination of the position of the catheter tip, enabling real-time manipulation of the imaging plane to always include the catheter tip. Finally, the improved catheter system will incorporate temperature sensors for monitoring ablation temperatures at the catheter tip. Rhythm abnormalities of the heart, where the heart beats too fast, affect millions of people in the United States. These abnormalities can cause substantial symptoms and/or death, and some can be cured by cauterizing (ablating) a small portion of the heart. Methods are being developed to use the real-time and high quality imaging of Magnetic Resonance Imaging to improve substantially the safety and efficacy of these ablations, as well as extend these methods to procedures involving other organ systems.